This invention relates to the production of dry black liquor solids and more particularly to a process and apparatus for producing such solids having a moisture content below five percent water by weight.
The production of black liquor as a by-product of the manufacture of products by a kraft process is well known, and the use of black liquor solids for combustion in recovery boilers is well documented in prior patents and technical literature. The interested reader is directed to Nelson U.S. Pat. No. 4,363,698; Liem U.S. Pat. No. 4,377,439; an article by Harrison et al entitled "Ultra-high-solids evaporation of black liquor" appearing in the February 1988 TAPPI Journal at pages 61 through 66; and an article by Hyoty et al entitled "High Solids black liquor combustion" appearing in the January 1988 TAPPI Journal at pages 108 through 111 for an understanding of work done prior to the present invention.
As has been recognized (and illustrated by a graph in the Harrison et al publication reproduced here as FIG. 1), black liquor solids have varying characteristics depending upon the water content. Heretofore, conventional practice has been to reduce the water content to a range such that the black liquor solids have a solids content in the range of about 60-65% by weight and then deliver the solids in a pumpable liquid into a recovery boiler where the water is driven off as steam and the solids are burned to provide heat energy and to recover chemicals used in the kraft process. Understandably, the heat energy necessary to drive off the water content imposes a significant burden on the energy balance of the process. Additionally, the balance between oxidizing and reducing conditions within a recovery boiler is delicate and the complications of driving water off and maintaining the necessary conditions often results in emissions from the boiler stack, with those emissions frequently carrying sulfur components and therefor contributing to odors and so called "acid rain" and other effects of pollution.
The Hyoty article, in particular, addresses this latter point, showing (in a graph reproduced here as FIG. 2) that the emission of sulfur is decreased markedly by an increase in the dry solids content of black liquor. As there shown and described, above about drops to essentially zero.
Efforts toward further decreasing the water content and increasing the efficiency of combustion while decreasing emissions have encountered the "sticky tarry" and "umpumpably viscous" ranges of black liquor solids, as shown by the graph of Harrison and described hereinafter. Prior to the present invention, there has been no viable process or apparatus disclosed which enables a leap over these ranges of water content or addresses the handling of black liquor solids as other than a liquid.